The invention relates to a reflective object detector which includes object detecting means which utilizes a signal such as ultrasonic wave, for example, for detecting the presence or absence of an object in response to the presence or absence of a reflected signal, and in particular, is directed to such detector which is capable of detecting an object located in an extended range of distances with an increased speed of determination.
When transmitting or receiving an ultrasonic wave, for example, the level of the received wave changes with the distance between the transmitting and the receiving points, even though the transmission level is maintained constant. Accordingly, if the ultrasonic transceiver has a reduced sensitivity as a whole, an object located at an increased distance results in a greatly reduced level of the received signal, preventing such object from being detected. Conversely, if an apparatus has a high sensitivity, the level of the received signal will rise to a very high value for measurement directed to reduced distances, causing the likelihood that noises induced by an electrical signal or the direct ultrasonic wave may be inadvertently detected. It is seen therefore that an adjustment of the sensitivity is required to enable the detection of an object over a broad range from a near to a far distance. By way of example, U.S. Pat. No. 4,542,489 discloses an arrangement in which a region to be monitored is divided into a plurality of sections, and the sensitivity is adjusted stepwise for each of the sections.
However, it is difficult to switch the sensitivity at high speed. For example, if an analog multiplexer is used to change resistors which determine the amplification factor of a receiver, an abnormal change in the gain may occur or switching noises may be developed during the switching operation. This prevents an accurate detection at a timing at which the sensitivity is switched. In other words, a continuous measurement is inhibited. For this reason, in the arrangement of the U.S. patent, the sensitivity is changed after each measurement. Thus, the ultrasonic wave is transmitted intermittently, and the sensitivity is adjusted at the interval of such transmission. This requires a plurality of measurements to monitor the entire region which extends over an extended range of distances. In order to prevent the influence of an ultrasonic wave which has been transmitted previously, it is necessary to provide a waiting time of a given length from the termination of one measurement until the next measurement can be initiated. The interval between successive measurements must be chosen to be a relatively high value which may be on the order of several tens of milliseconds. Since a number of waiting times is required to monitor the entire region, the overall time required for the measurement increases. Where a vehicle or an object moves relatively rapidly, a detection lag is likely to occur. In particular, if a plurality of detector units are located at spaced points, and measurements are made at successively different locations so as to cover a broad range, the total time required for the measurement will be very long.